X-ray radiography is commonly utilized to visualize a target object, for example bone or tissue. An x-ray source generates x-rays which penetrate the target object, and are transmitted therethrough. Variations in composition and thickness in different regions of the target object are revealed, as a result of the differential absorption of x-rays in the different regions.
In film-based radiography systems, the x-rays are registered on a photosensitive film placed behind the target object. The variations in the composition and thickness of the target object appear as contrasts, e.g. dark and light areas, on the film. In digital x-ray radiography systems, a digital x-ray detection system (e.g. a flat panel detector) is connected to a digital image processor, which processes the detection signals to generate digital images of the target object.
As x-rays traverse a target object, the x-ray photons undergo various interactions (e.g., absorption and scattering) with the atoms forming the target object. As a consequence, the x-rays are attenuated exponentially, according to the following formula:I=I0e−μx,where I is the intensity of x-rays that have passed through the object, I0 is the intensity of x-rays before passing through the object, μ is the energy-dependent attenuation coefficient of the object at a given x-ray energy, and x is the thickness of the object.
In radiography in general, and in mammography in particular, the x-ray spectrum must be precisely controlled, in order to optimize the quality of radiographic image, as well as the diagnostic procedure for the patient. The goal is to obtain an optimal image quality for the radiograph, while subjecting the patient to the lowest possible dose. Regarding image quality, it is desirable that an x-ray radiograph provide high spatial resolution and high contrast-to-noise ratio. In film mammography, an additional requirement is that the film be properly darkened. In mammography, the tissues under examination are very similar in structure and composition, and therefore precisely controlling the x-ray exposure parameters is critical, in order to obtain an image having an adequate contrast and resolution.
At present, a known technique for optimizing the x-ray spectrum for an x-ray radiography procedure is the acquisition of a single pre-exposure at the beginning of the exposure, at a constant kVp level. The pre-exposure takes place during a time period that is short compared to the total exposure time. Information from sensors which detect pre-exposure radiation is used to adjust the x-ray exposure parameters (e.g. operating voltage, operating current, focal spot size) of the x-ray tube for the main exposure. This procedure (sometimes referred to as the “pre-pulse” method) provides better control of the parameters, as compared to other known methods in which an estimate of the thickness and density of the target object is all that is used when setting parameters such as the operating voltage and the operating current, for the entire exposure period.
The above-described “pre-pulse” procedure has a number of limitations, however, which arise from inaccuracies that occur when optimal x-ray spectra and optimal x-ray exposure parameters are predicted using a sensor signal at a single, constant kVp level. Pre-pulse sensor information at constant kVp is often unreliable, because of sensor sensitivity drift. The overall attenuation measurement result at constant kVp is strongly affected by the scattered radiation. Because of these limitations, the pre-pulse method currently known in the art does not always allow a maximum image contrast-to-noise ratio to be attained, at a minimum level of radiation dose to the patient.
It is therefore desirable to provide an x-ray exposure control technique that overcomes the limitations (described above) of currently used methods of exposure control in radiography, including mammography.